1. Field of the Invention
The invention relates generally to xenon short-arc ceramic lamps and specifically to such lamps which incorporate a spherical-elliptical reflector combination in a compound system to improve efficiency.
2. Description of the Prior Art
Short arc lamps provide intense point sources of light that allow light collection in reflectors for applications in medical endoscopes, instrumentation and projection. Also, short arc lamps are used in industrial endoscopes, for example in the inspection of jet engine interiors.
A typical short arc lamp comprises an anode and a cathode positioned along the longitudinal axis of a cylindrical, sealed concave chamber that contains a gas pressurized to several atmospheres. U.S. Pat. No. 4,633,128, issued Dec. 30, 1986, to Roy D. Roberts, the present inventor, and Robert L. Miner, describes such a short arc lamp in which a copper sleeve member is attached to the reflecting wall to conduct heat from the reflecting wall through to the exterior wall and eventually to circulating ambient air.
U.S. Pat. No. 4,305,099, describes a light collection system for projectors, such as light valve projectors, which have a compound reflector associated with an arc lamp. The compound reflector includes an ellipsoidal reflector positioned to collect a portion of the light from the arc lamp and reflect a direct image of the light in a beam to an image forming plane of the projector and a spherical reflector positioned to collect another portion of the light from the arc lamp and reflect it back through the gap of the arc lamp to the ellipsoidal reflector to be reflected as a secondary image of the light from the lamp in the beam. The ellipsoidal and spherical reflectors are formed as full, uninterrupted surfaces of revolution. To provide uniform light distribution, the beam is directed through a pair of spaced lens plates, each having corresponding arrays, in rows and columns, of rectangular lenticules. The adjacent focus of the ellipsoidal reflector is centered in the arc, while the center of curvature of the spherical reflector, in order to avoid transmission loss through the arc, is displaced to a portion of the gap of the lamp which is relatively free of the arc. For maximum light efficiency, the direct image is focused just to one side, and the secondary image is focused just to the other side of the image forming plane. Such patents are all incorporated herein by reference.
Conventional lamps with parabolic collector/reflectors have the advantage of good collection and distribution efficiency when used in conjunction with a lens for focusing. However, such combinations can be too expensive for many applications. Conventional lamps with elliptical collector/reflectors have a different kind of problem. In order to collect a large polar angle of the lamp output, a wide spread of arc magnifications are automatically generated at the second focus. The rays with the smallest angles have the largest magnification. And the rays with the largest angles have the smallest magnification.
The collection efficiency of conventional elliptical collector/reflectors is good, but the distribution efficiency is often poor. In a compound reflector geometry that combines reflector types, the elliptical part is usually a rather shallow dish that provides a small spread of arc magnifications over a select spread of ray angles. But the polar angle collection of such a lamp's output is rather poor from the ellipse.